This invention relates to apparatus for the protection of loads connected with an electrical distribution system and for indication of the status and capability of the surge suppressor system. The invention relates particularly to such apparatus suitable for incorporation within household and commercial wiring devices.
It is beneficial to have a surge suppressor to protect loads against some voltage transients that do not trip the circuit breaker of a distribution circuit. This is particularly so in the case of relatively delicate electronic equipment such as video recorders, personal computers, and other valuable household and commercial appliances. It is also desirable to have an indication that the surge suppressor is in good working order.
It is recognized that considerable prior art relates to the protection of equipment against overvoltages with an indication of the occurrence of an overvoltage. Many protection systems are fairly complex, expensive and bulky. The present invention is primarily directed to a system that is simple, inexpensive and compact and is used, normally, for local load protection and is in addition to conventional distribution circuit protection devices such as circuit breakers.
In accordance with a prior art system for applications addressed by the present invention, a surge suppressor unit is provided in a receptacle into which the line cord for the load is plugged. It includes a suppressor circuit branch, for connection between the hot and neutral conductors of the distribution system, that includes a fuse and a metal oxide varistor in series. Such a surge suppressor circuit branch is connected between the hot and neutral conductors of a distribution system; other conductor pairs of a three wire system may be protected as well. The fuse is selected to have an interruption, for fire safety, if a failure of the varistor occurs, such as shorting due to an overvoltage. In such an arrangement it is known to utilize an indicator device, such as a green light-emitting diode, that is powered only if the fuse is intact. This light will light and therefore indicate the power is on and the fuse is present. If the indicator light is not on, this indicates the power is not on or the fuse has blown. It would be desirable to know which of these two events is the case.
Systems are known in which the one light used which is energized when everything is correct is arranged to blink if surge protection is lost. This requires an extra timing circuit portion and its attendant cost and complexity.
In accordance with another version of such equipment, there is provision to provide a buzzer alarm when protection is lost and the power is still on. This, too, incurs additional cost and complexity.
It is desirable to provide an alternative system that gives positive indication of whether the fuse is or is not operable by two distinct indicator devices in a relatively simple and inexpensive arrangement.
Therefore, in accordance with one aspect of the present invention, there is provided an electrical surge suppressor and indicator system that includes, at least, a first surge suppressor circuit branch for connection between two line conductors and including a fuse and a first varistor in series. A first indicator circuit portion indicates power is on and surge protection is present and includes a first indicator device energized by the line conductors if the fuse is intact. A second indicator circuit portion indicates power is on and surge protection is lacking and includes a second indicator device energized by the line conductors if the fuse has been interrupted.
In certain embodiments of the invention the first indicator device is a green light-emitting diode, and the second indicator device is a red light-emitting diode. Each light-emitting diode is in series with current-limiting resistance. In the case of the green light-emitting diode, which is energized when the fuse is intact, it is in series with a resistor and the red light-emitting diode is in series with that same resistance, plus an additional resistance. The first and second indicator circuit portions may both include the first resistor. The second indication circuit would then include a separate, second resistor that is not in the first indicator circuit portion.
When light emitting diodes (LEDs) are used as the indicator devices, they can be used in different ways to yield different characteristics in the information indicated. In one form of connection, the LEDs are likepoled; each is in inverse parallel relationship with another, standard, diode having a lower forward voltage drop. When the fuse is intact and the green LED is energized, there is some current drawn through the second circuit portion containing the red LED. With appropriate resistance present (so both the red and green are of approximate equal brightness when fully on), that current is sufficient to energize the red LED just slightly producing a perceptible but dim red glow while the green LED is fully energized and considerably brighter. This allows one to readily confirm the operability of both LEDs when power is on.
If one does not wish any perceptible output from the red LED when the green is energized, it is merely necessary to change the polarity of one of the LEDs, such as the green LED, and its respective secondary diode.
It is desirable in the systems of interest where there is surge protection between the hot and ground conductors to also have surge protection between the hot and neutral conductors as well as between the neutral and ground conductors. To do this, prior devices have had three separate varistor devices in appropriate circuit branches between the indicated conductors. They include a single fuse with respective varistors in branches between the hot and neutral conductors and between the hot and ground conductors. A third varistor, not in series with a fuse, is located between the neutral and ground conductors; if it fails its failure is not indicated externally.
A substantial degree of protection between the ground conductor and the neutral conductor is provided by the series connection of the first two varistor devices. In accordance with a further aspect of the invention, two similar varistor metal oxide disks are packaged as a unit within a single insulating casing and are provided with three terminals--one on each side of the outside surfaces, and a third centertap between the two disks. Therefore, with minimal bulk this provides effectively three varistors for protection between each pair of conductors. Any varistor failure, in contrast to the prior art, is indicated externally, such as by the dual indicator system described above. Such a compact, center-tapped, arrangement of two varistor disks is also believed novel and useful in other applications.
The compactness and economy of apparatus in accordance with the present invention allows its convenient use within ordinary wiring devices, such as receptacles for plugging in various kinds of appliances which are thus protected.